Electrical submersible pumps (“ESP”) of the type concerned herein are used for pumping high volumes of well fluid. The pump of this type has at least one electrical motor that normally is a three-phase, AC motor. The motor drives a centrifugal pump containing a large number of stages, each stage comprising an impeller and a diffuser. The motor is filled with a dielectric lubricant or oil. A seal section connects between the pump and the motor for equalizing the pressure of the lubricant contained within the motor with the hydrostatic pressure on the exterior. The seal section is filled with oil that communicates with the oil in the motor. Two or more electrical motors may be connected in tandem to drive large pumps. The connecting adapter between the motors preferably has communication passages so that the oil in each motor communicates with oil in the other motor or motors.
Techniques are employed to reduce any air pockets that may be trapped within the motor or seal seation, because air within the oil is detrimental. For convenience, the motor and seal section are referred to at times herein as subs of an ESP assembly. Normally the subs are filled with oil at a manufacturer's facility, then caps are secured to the ends to retain the oil. During filling, the sub is inclined and a vacuum hose connected to a port at the head of the sub. An oil injection hose is connected to a fill port on the base of the sub. After evacuation, the operator pumps oil into the sub, then secures sealing caps to the ends.
When at the wellsite, normally a workover rig with a derrick will be present for running the ESP on the lower end of a string of production tubing. The operator picks up the motor and suspends it vertically over the wellbore. The operator removes the cap from the seal section and lowers it onto the head of the motor. During this process some oil will usually leak out from the seal section. The drive shaft in the seal section has a lower splined end that is located within a cavity in the base of the seal section. The upper end of the cavity is closed by a shaft bushing which restricts oil from flowing down the central cavity. Once the shipping cap is removed, the lower end is open because it must receive the upper end of the drive shaft of the motor. This downward facing cavity traps air when the motor and seal section are connected together.
After the connection is made up, the operator tops up the oil in the seal section by pumping oil up through an upper port in the motor. Air, including the trapped air in the cavity, is vented through a port at the top of the lower chamber of the seal section.
Removing trapped air is more difficult for the interconnection between tandem motors because the components within the upper motor tend to restrict upward air migration during operation. The interconnection between tandem motors is quite similar to the interconnection between the motor and the seal section. Some upper tandem motors may have a check valve to restrict downward flow of oil after the shipping cap is removed and before make-up, however, the trapped air pocket will still exist.
In some instances, the operator may connect a vacuum pump to an upper port in the upper tandem motor when topping up the oil after make up of the upper tandem motor to the lower tandem motor. While this wellsite vacuum filling procedure may reduce or eliminate trapped air pockets at the interconnection, it takes more time to use the vacuum to top up oil than simply pumping oil in a lower port and venting at an upper port. The additional time is costly because of the cost of the workover rig. Also, vacuum filling of tandem motors at the wellsite is difficult to accomplish in cold climates.